Animals generate heat during physical exertion, which can lead to a detrimental escalation in body temperature. To regulate that escalation, human beings, unlike some other animals bearing fur, perspire during exercise. Their perspiration then may evaporate from bare skin and thereby aid in cooling the body. This form of sweat is known as eccrine perspiration, and involves the eccrine sweat glands, which are distributed throughout the skin surface of the body. Thermoregulatory sweat from eccrine sweat glands includes water and Sodium Chloride (common table salt), which may be, to some degree, reabsorbed by the body to decrease salt loss.
In modern athletics, hyperthermia during intense competition is a serious issue, both in terms of health and athletic performance. Although natural thermoregulatory sweating may be helpful in controlling overheating, it may be ineffective, or insufficiently effective, for a variety of reasons. The exercise environment may be too hot and/or too humid to permit effective evaporative cooling by sweat. The athlete may even become uncomfortable with the dampness of his or her own body, and his or her garments saturated with sweat, and “towel off” or change clothing during exercise. Conversely, some athletes, particularly in marathon running, may not mind the damp sensation, and in fact douse themselves with water in an effort to cool down more greatly than with their own sweat, alone, because the water may be temporarily cooler than the athlete's sweat. However, this added relief may be temporary, and may come with the disadvantages of sodium and other electrolyte loss as those agents are washed from the body, closing eccrine sweat glands, and other factors causing decreased sweating efficacy.
Some freezable gels have been created in pads that aid in locally cooling painful or injured parts of an athlete's body. These pads may include compression bands, to further reduce swelling. These pads tend to reduce or restrict cooling by sweat, because sweat and other glands and pores may be closed by lower temperatures. Some “cooling” gels have been brought to market that provide a cooling sensation for post-exercise pain relief. Typically these formulations include menthol. The U.S. government, and other entities, have created artificial sweat formulations, which closely mimic the composition of eccrine sweat, for the purpose of in vitro or other product testing.
In recent decades, progress has been made in fundamental research concerning the thermal topography of the human body and the distribution of eccrine sweating during exercise. In The topography of eccrine sweating in humans during exercise, Cotter J D, Patterson M J, Taylor N A, Eur. J. Appl. Physiol. 71(6):549-54 (1995), the authors reported steady state sweat rates of men during stressful exercise (cycling) and heat exposure. The scapula, forearms, hands, stomach and lower back sweat rates were relatively great (and are listed here in descending order) and exceeded sweat rates for the chest, upper arms, calves and thighs. The authors noted that their results differed from measurements of sweat response taken during resting heat stress, with a subject laying in the supine position. In that scenario, sweat onset in a caudal-to-rostral pattern, whereas in the author's work, onset was similar across most regions. Other research has indicated that there are some important differences between the sexes in regional sweating rates. For example, sweating rates are relatively low at the breasts in aerobically trained, exercising female subjects, when compared to aerobically trained, exercising males. In addition, sweating rates were relatively great in the arms and hands of females, in comparison to males. But upper back regional sweating was relatively great in both sexes. See Smith C J, Havenith, G, Body Mapping of Sweating Patterns in Athletes, Med. Sci. Sports Exerc., 44(12):2350-2361 (2012). Still other research has revealed thermal images for athletes prior to exercise, upon reaching age-predicted maximal heart rate, and during recovery from exercise. This work shows a tendency for relatively great cooling effects in the thighs, forearms and torso of trained male participants. See Thermal Imaging of Cutaneous Temperature Modifications, Merla A, Mattei P A, Di Donato L, and Romani G L, Annals of Biomed. Engineering, Vol. 38, No. 1 (Jan. 2010).
It is the inventor's new, working hypothesis that a variety of specific factors contribute to a cooling adaptation response by humans, partly exhibited in the results of the research discussed above. First, the effective thermal exchange rate for net body cooling differs at different skin regions of the body. Areas lined with adipose tissue, or with other forms of insulation, and without thermal communication to relatively great blood flow are less effectively used by the body as a cooling surface. As a result, the human body makes less investment in a cooling apparatus in those areas, and some of those areas may be thought of as bearing a thermoregulatory “penalty” in the sense that temperature regulation efforts (such as sweating) bear a cost too great to be justified (such as simple loss of water and electrolytes, without significant enough cooling) in comparison to other skin areas. In addition, skin regions of the body with high local blood flow and low local underlying body volume are more effective for heat transfer (and, therefore, as a thermal regulation device) due to the cube of a length relating to volume of an object and square relating to surface area of an object principle of engineering, biology and physics, generally (“cube-square law”). Overall, the effect of gravity pulling sweat, as a cooling fluid, from one region to another as it evaporates changes the ideal distribution of sweat gland activity upon exercise, in comparison to what it would be in different (e.g., supine) positions or in a gravity-free environment. Other factors, such as the diffusion characteristics of the surface of skin in each region, or the flow of fluid through fabrics, and the present conformation of the skin and fabrics, the characteristics of a distributed cooling fluid or matrix, as well as atmospheric and other conditions, also may modify the ideal distribution patterns of a cooling fluid. The principles of this hypothesis, its refinements, and others which have followed and may come, may impact some, but not all, of the design factors set forth in this application.
It should be understood that the disclosures in this application related to the background of the invention, such as those in, but not limited to those in, this “background” section, do not necessarily set forth prior art or other known aspects exclusively, and may instead include art that was invented concurrently or after the present inventions and details of the inventor's own discoveries and work and work results. Thus, nothing in this background section should be construed as an admission of “Prior Art.” It should also be understood that, for convenience and readability, this application may set forth particular pronouns and other linguistic qualifiers of various specific gender and number, but, where this occurs, all other logically possible gender and number alternatives should also be read in as both conjunctive and alternative statements, as if equally, separately set forth therein.